"Teacup" Dogs Don't Exist!

Teacup dogs, also known as "micro dogs" and "pocket pups", is a marketing term designed to appeal to people who want super small dogs... usually they are under four pounds. The world's official smallest dog breed is the Chihuahua, so they are often bred to be even smaller. Yorkshire Terriers (Yorkies), Malteses, Pomeranians, and Shih Tzus are also often bred to be smaller than the breed standard to appeal to certain buyers.

They are very popular with small dog owners and with good reason- they are beyond adorable, but unfortunately, they are usually the product of unethical breeding practices. Because the emphasis is on size rather than health and adhering to the breed standard, they can suffer from severe health issues. 

Naturally, any super small dog can suffer from hypoglycemia because their size doesn't allow for a lot of reserves, so you will always have to make sure that their blood sugar doesn't get too low.

Breeding for that adorable "dome like" head can lead to hydrocephalus (water on the brain), which is created by extra pressure within the skull. This can damage the brain and lead to blindness and other neurological symptoms. Surgery can be done to give that fluid a pathway away from the brain.

Liver shunts are abnormal blood vessels that prevent blood from entering the liver, in "teacup" dogs, this is frequently caused by numerous tiny vessels that cannot be surgically corrected- medication can help, but not cure the condition.

They tend to be excessively fragile and prone to hypothermia due to their lack of body size.

Any small dog can also be prone to a collapsing trachea, dental disease, heart disease, and orthopedic issues (like a luxating patella), but I would consider those to be less specific to "teacup" dogs and more general health concerns to be aware of as a small dog owner.